On a grading plan, the proposed-grade contour often represents finish grade — the surface at the end of construction. Subgrade is one layer below: the compacted soil that supports whatever paving, slab, or structural fill goes on top.
Subgrade preparation is its own scope item on most sitework bids. Excavate to the design elevation, compact to 95-100% Proctor density (per the geotech report), then proof roll with a loaded dump truck to identify soft spots. Repair soft spots with over-excavation and structural fill before placing the next layer.
Subgrade depth depends on the load above. Light parking lots may have 12-18 inches of structural fill plus base course over subgrade; heavy-duty roads have 24-36+ inches. Foundation subgrade is per the structural engineer's spec, typically excavated to bearing-stratum depth.